1. Field of the Invention
The present invention relates to an image forming apparatus configured to properly form an image on a sheet of paper even when papers are not properly loaded on a sheet loading section.
2. Description of Related Art
Generally, an image forming apparatus has a sheet feeding section for feeding papers into the apparatus. The sheet feeding section includes a sheet loading section loaded with a plurality of papers by loading and a sheet feed roller for feeding the papers loaded on the sheet loading section into the apparatus. When image forming, papers loaded on the sheet loading section are fed one by one from the topmost one to the apparatus by a feeding action of rotating the sheet feed roller, and then an image is formed on a surface of the fed paper.
Paper replenishment to the sheet loading section is made by a user of the image forming apparatus. Papers on the sheet loading section are not necessarily supplied properly on the sheet loading section, and the loaded state is sometimes not preferable. Further, there is a case that a paper to be fed by the feeding action gets stuck or that the sheet feed roller performing the feeding action makes a slip with the paper. In those cases, even if the feeding action of the sheet feed roller gets started, there may be a sheet feeding failure that the paper is not fed into the apparatus. Some image forming apparatuses are configured to perform a feed retry control of once stopping the started feeding action and retrying the feeding action when the sheet feeding failure occurs. The paper that failed to be fed in the initial feeding action is sometimes fed properly by the feed retry control.
Judgement of whether or not the sheet feeding failure occurs is, for example, made by a sheet sensor provided on the way of a sheet conveying path in the apparatus for detecting arrival and passing of the paper. Specifically, at a usual time when sheet feeding is properly performed, the fed paper arrives at a detection point of the sheet sensor within a predetermined time from start of the feeding action. On the contrary, when the sheet feeding failure occurs, a period of time from start of the feeding action to arrival of the paper at the detection point of the sheet sensor takes longer than usual arrival time. Therefore, in a case that arrival of the paper is not detected by the sheet sensor for a period of time longer than the usual arrival time since start of the feeding action, it is concluded that the sheet feeding failure has occurred and thus the feed retry control is performed.
Japanese Patent Application Publication JP-A-2013-119469 teaches that when the sheet feeding failure occurs in the feeding action, the feed retry control is performed in such a manner that after rotation of the sheet feed roller is stopped and the sheet loading section is once lowered, the section is raised again so that the rotation of the roller is restricted. The feeding action is thus restarted after descent and ascent of the loading section, so that it is possible to properly feed the paper of which a front end has got stuck during the previous feeding action before the feed retry control.
Herein, when papers are added disorderly on the topmost one of papers which have already been loaded properly on the sheet loading section, the added papers are sometimes positioned not to be in contact with the sheet feed roller. Specifically, a person who adds the papers is apt to add the papers on a position apart from the sheet feed roller of the sheet loading section. When the feeding action gets started in this state, the sheet feed roller firstly feeds the topmost one of the initially loaded papers loaded on the sheet loading section and placed below the added papers.
The added papers are loaded on the paper which has started to be fed, and accordingly, the added papers are moved toward the sheet feed roller in accordance with the feeding action of the paper which has started to be fed. When the added papers thus move and arrive at the sheet feed roller, the topmost one of the added papers comes to contact with the sheet feed roller. Namely, the topmost added paper is fed into the apparatus instead of a paper placed beneath the initially fed paper. The paper initially fed and located beneath the added paper is not in contact with the sheet feed roller since the added paper arrives at the roller instead. Therefore, the paper initially fed and located beneath the added paper has been fed but stopped at a position when the added paper starts to be fed.
When the added papers are disorderly supplied on the sheet loading section on which papers have already been loaded, the feed retry control is performed in the above mentioned prior arts and others. The feed retry control is performed because an arrival time from start of the feeding action to arrival of the paper at a detection point of the sheet sensor becomes longer than the usual arrival time by the period of time from start of the feeding action to the time when the topmost one of the added papers comes to contact with the sheet feed roller. In a case that the feed retry control is performed, the topmost one of the fed added papers is detected by the sheet sensor, and the image is properly formed on that detected paper.
However, beneath the added papers, the papers which have been initially loaded on the sheet loading section are present. The topmost one of those papers initially loaded on the sheet loading section is fed but stopped on the way of the sheet conveying path. Therefore, for example, prior arts have the following problems.
Feeding of a paper after all the added papers are fed restarts from a position on which the paper has been fed on the conveying path. Therefore, when images are continuously formed on the last one of the added papers and the paper having been fed on the way of the conveying path, a front end and a rear end of those continuously fed papers could be overlapped each other. In such a double-feeding (multi-feeding), size error and jam could be detected, so that the feeding action could be stopped in the image forming apparatus. Even when the front and rear ends of the successive papers are not overlapped, the apparatus could wrongly detect size error and jam due to a too narrow sheet interval.
For example, Japanese Patent Application Publication No. JP-A-2010-265051 teaches the technique of detecting multi-feeding of papers by use of an ultrasonic sensor. Specifically, it is possible to ensure a preferable sheet interval by detecting a state of multi-feeding and by carrying out conveyance of papers such that the multi-feeding is resolved. However, a sensor for detecting the multi-feeding is required and therefore an image forming apparatus cannot be provided with low cost.
The present invention has been made to solve the above problem of the prior arts. Namely, the present invention has a purpose to provide an image forming apparatus capable of preventing a problem that a sheet interval is not sufficiently ensured during continuous printing due to disorderly supplied papers.